1. Field of The Invention
The present invention relates generality to an apparatus for monitoring air leakage into a fuel supply system for an internal combustion engine. More particularly, the invention is directed to a purging operation failure detection system that operates so as detect a failure of an air-fuel mixture control system caused by variation in pressure in a fuel supply system resulting from air leakage thereinto.
2. Description of The Prior Art
Japanese Utility Model First Publication No. 2-26754 discloses a system which detects a negative pressure level in a purge passage communicating between a canister connected to a fuel tank and an intake passage of an engine, and determines that a failure in a purging operation (i.e., air leakage into a fuel supply system) occurs when the negative pressure level in the purge passage is lower than that in the intake passage.
In the prior art system, the pressure in the intake passage tends to vary greatly directly following a change engine speed. The variation in pressure in the purge passage due to the pressure variation in the intake passage is, however, delayed because of the large volume of the fuel tank, with the result being that the system mistakenly determines that a failure occurs in the purging operation.
For avoiding the above drawback, a system may be proposed which blocks fluid communication between the canister and an inlet port of the intake passage through the purge passage, and determines that a failure has occurred in the purging operation caused by an air leakage into a fuel supply system when a reduction rate of negative pressure in the fuel supply system exceeds a threshold level.
The above system, however, raises the following drawback, the volume of a line of the fuel supply system in which pressure is to be measured varies dependent upon the amount of fuel remaining in the fuel tank, a variation rate of the pressure in the fuel supply system may represent different values even if the amount of air leaking into the fuel supply system is constant. In order to avoid this drawback, the pressure variation rate may be compensated based on the amount of the remaining fuel detected by a fuel level sensor. It is, however, difficult to determine the volume of the fuel supply system in which pressure is measured because the fuel tank has a complex shape. Additionally, the fuel level sensor must be designed to have an explosion-proof construction, resulting in the total costs of the system being increased.